1. Field of the Invention
The present invention is generally related to photolithography systems, and more particularly, to maintaining an oxygen-purged optical path through a reticle and pellicle.
2. Background Art
In the fabrication of integrated circuits, photolithographic and projection printing techniques are used. In photolithography, an image contained on a reticle is projected onto a wafer having a photosensitive resist thereon. The reticle or mask is used to transfer a desired image onto the silicon wafer. The semiconductor wafer surface is coated with photosensitive resist so that an image is etched thereon. A pellicle may be used in combination with the reticle to protect the reticle surface from damage. The pellicle is traditionally mounted on a solid frame to the reticle.
Some wavelengths of light used in photolithography are sensitive to absorption by atmospheric oxygen. Hence, when such oxygen-sensitive light wavelengths are used in photolithography, they must be transmitted through an oxygen-purged atmosphere.
A photolithography system is typically located in a clean room environment. In some situations, the ambient atmosphere of the clean room cannot be purged of oxygen because this may cause other problems with the photolithography process. For instance, a laser interferometer used in a lithography system may be sensitive to changes in the index of refraction of the air, which may occur with a change to an oxygen-free atmosphere. Hence, the oxygen-free environment may have to be restricted to less than the entire lithography system. What is needed is a transmission medium for light wavelengths that have high absorption in an oxygen-containing environment.
A pellicle is generally mounted on a frame opposite a corresponding reticle. Hence, an air gap may exist between the reticle and pellicle. What is needed is a transmission medium through the reticle-to-pellicle air gap for light wavelengths that have high absorption in an oxygen-containing environment.